The invention relates to a method for evaluating the condition of a fluidized bed reactor by determining static characteristic during polymerization. Specifically, the invention relates to methods for determining instantaneous static levels at the distributor plate of fluidized bed reactors. The invention especially relates to measuring static levels during metallocene-catalyzed polymerizations. The static measurements provide an indication of major continuity disturbances in the fluidized bed gas phase reactor.
In the gas phase process for production of polyethylene, gaseous ethylene, hydrogen, co-monomer and other raw materials are converted to solid polyolefin product in a commercial gas phase reactor, which consists of a fluidized bed reactor, a compressor and a cooler. The reaction is maintained in a suspended two-phase mixture of granular polyethylene and gaseous reactants by the fluidizing gas which is passed through a distributor plate near the bottom o the reactor. The reactor is normally constructed of carbon steel and rated for operation at pressures up to about 50 bars (or about 3.1 MPa). Catalyst is injected into the fluidized bed. Heat of reaction is transferred to the circulating gas stream. This gas stream is compressed and cooled in the external recycle line and then is reintroduced into the bottom of the reactor wherein it passes through the distributor plate. Make-up feedstreams are added to maintain the desired reactant concentrations. Operation of the reactor is critically dependent upon good mixing for uniform reactor conditions and heat removal. The process must be controllable, capable of high production rate and free from upsets due to particle overheating.
The internal surfaces of the reactor are also composed of carbon steel, and in a normal state appear as a plain, uncoated metal. But reactors that have been in service for any length of time typically have a thin coating of polymer adhered to the interior. The coating is usually thin and relatively clear, making its presence difficult to detect visually. Consequently, the wall coating is detected with a thickness meter, which indicates a typical thickness on the order of about 10 to 50 microns. This coating has a significant effect on the operability of the reactor through its affect on the static charging characteristics of the fluid bed.
The major detriment to good operability is the problematic and frequent xe2x80x9csheetingxe2x80x9d events. Sheeting is associated with the undesirable accumulation of polymer along the reactor wall in the zone occupied by the main fluid bed. This accumulation is believed to be associated with xe2x80x9cfinesxe2x80x9d, the fine particles less that 100-200 microns. These fines are more influenced by static electrical forces due to their larger surface area relative to their mass, a counter-play of static versus inertial forces.
The stagnation of the resin particles results in a significant reduction in the heat transfer from the nascent particles, precisely at the point in their growth that heat generation per unit surface area is at a maximum. The next result is an interplay of forces which results in particle overheating, melting and agglomeration with adjacent particles, both overheated and normal type particles. The net result is the formation of sheets along the vessel wall. Progressive cycles in this process eventually result in the growth of the sheet and its falling into the fluid bed. These sheets interrupt fluidization, circulation of gas and withdrawal of the product from the reactor requiring a reactor shutdown for removal.
U.S. Pat. Nos. 4,803,251 and 5,391,657 describe a static mechanism as a contributor to the sheeting phenomena whereby catalyst and resin particles adhere to the reactor walls due to static forces. Numerous causes for static charge exist. Among them are generation due to frictional electrification of dissimilar materials, limited static dissipation, introduction to the process of minute quantities of prostatic agents, and excessive catalyst activities. Strong correlation exists between sheeting and the presence of excess static charges, either negative or positive. The critical level for sheet formation is not a fixed value, but is a complex function dependent on variables including resin sintering temperature, operating temperature, drag forces in the fluid bed, resin particle size distribution and recycle gas composition.
Sudden changes in static levels followed closely by deviation in temperatures at the reactor wall is evidence of a sheeting occurrence. These temperature deviations are either high or low. Low temperatures indicate particle adhesion causing an insulating effect from the bed temperature and are commonly referred to as xe2x80x9ccold bandsxe2x80x9d. High deviations indicate reactions are taking place in zones of limited heat transfer and are commonly referred to as xe2x80x9chot spotsxe2x80x9d.
Another undesirable place where fines accumulate is the disengaging section of the reactor termed the expanded section, which consists of a region of expanded cross-section above the reaction zone. The function of the expanded section is to reduce the velocity of the fluidizing gas in order to minimize the entrainment of fine particles in the gas leaving the reactor. The entrained fines concentrate in the regions of lower gas velocity. The intention is to use this concentration of particles to xe2x80x9cwashxe2x80x9d the inclined portion of the expanded section by the downward sliding of these particles onto each other and back into the fluid bed section of the reactor. However, the increased loading of polymer in the expanded section may increase the heat load in an area having low heat transfer capability due to the loss of fluidization and particle mixing in this zone. The resulting excess of heat generation relative to heat removal leads to the melting and fusing of polymer into sheets. As the sheets increase in mass, gravity pulls the xe2x80x9cdomexe2x80x9d sheets into the main reactor section. The impact on reactor operation can be even more serious because the dome sheets generally possess a large surface area and are thicker than wall sheets. In extreme cases, a large dome sheet causes total blockage of the distributor plate and the formation of a single large reactor agglomerate, or chunk. It is thought that increased polymer loadings in the expanded section results initially from statically charged fines first clinging in this area of reduced gas velocities. That is, it is hypothesized that static generation occurs elsewhere in the reactor and/or recycle system and the consequence thereof is dome sheeting and/or chunking.
Because of the significant manufacturing and operating costs associated with the occurrence of sheeting-related events, mechanisms to control xe2x80x9csheetingxe2x80x9d in fluidized bed reactors are continuing areas of investigation in the industry (for example, see U.S. Pat. Nos. 5,436,304 and 5,405,922). Another technique that is directed to reducing sheeting involves the introduction of water into the reactor at a site proximate the reactor walls in an amount sufficient to maintain the electrostatic levels at the site of possible sheet formation at levels which avoid sheeting without substantially altering the effectiveness of the catalyst(s) employed (U.S. Pat. No. 4,855,370, which in herein incorporated by reference in it entirety). Various methods described involve monitoring static charges near the reactor wall in regions that display a high propensity of sheeting. For example, static levels are controlled within a predetermined range by introducing a static control agent into the reactor (U.S. Pat. Nos. 4,803,251 and 5,391,657). In these cases, static charge is measured using static voltage indicators such as voltage probes or electrodes, and measurements are taken at or near the reactor wall, at or below a site commonly plagued by sheet formation.
Static level; in a fluidized bed are typically measured and determined using static probes. Conventional static probes use a rod with a ball on the nd of the probe to determine the static level by measuring voltage in the fluidized bed tar. The ball-type probe is usually inserted into the reactor. EP 0604990 and U.S. Pat. No. 6,008,662 both describe in-reactor ball-type static probe; (see also, U.S. Pat. Nos. 4,532,311; 4,792,592; 4,855,370. However, measurements taken with the conventional static probes do not indicate the origin of the static, which is important in assessing operability of the reactor. More importantly, it is difficult to locate conventional static probe a certain position throughout the reactor and/or recycle system. For example, it is difficult to locate conventional static probes at the distributor plate. Furthermore, conventional static probes placed at conventional locations are ineffective in detecting instances of high static generation during polymerizations with metallocene catalyst systems. With ineffective detection, sheeting incidents occur without any apparent wanting of the onset thereof. Conversely, effective early detection allows the performance of corrective operations and actions to avoid or minimize sheet formation.
Based on Applicants hypothesis that the substantial static originates at the distributor plate, the present invention is directed to systems and methods of determining a reactor wall condition, including the reactor dome condition, using a static detector that is located at the distributor plate. The novel detector (which measures current flow through the distributor plate) provides more information and improved sensitivity as compared to conventional static detection methods. Alternatively, the present invention provides a static detection system that employs radio frequency. Further, the methods of the present invention indicate a condition of the reactor wall, and more particularly indicate major continuity disturbances in an operating reactor.
The present invention is directed to systems and methods that determine a reactor wall condition by measuring static levels therein.
Measurement of the static levels in the reactor may be accomplished by one of several means. First, the invention relates to a method for determining instantaneous static levels or the development of static charges by using a static detector comprised of an electrically isolated distributor plate cap. With the novel detector, it has been surprisingly discovered that for metallocene-catalyzed polymerizations, static charges first develop at or near the distributor plate. By careful monitoring the charges at the distributor plate, changes in the charges that are indicative of changes in the reactor can be detected early allowing more time to take corrective measures to void or minimize reactor sheeting and/or chunking incidents.
In an alternative embodiment, static levels can be measured using a radio frequency detector. In this embodiment, changes in the radio frequencies in the reactor are measured and used to predict changes in reactor conditions.
In yet another embodiment of the present invention, the static level measured is employed in a non-linear dynamic calculation to determine the reactor wall condition, and more particularly to predict the onset of a major continuity disturbance.
The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present invention.